The prior art discloses systems for which the analog signals are predetermined at the time at which the system is designed (in this case, the parameters of the signals are fixed). There are also known systems for which the analog signals are generated on request (in this case, the parameters are sent to the system in real time).
In the case of predetermined signals, the following systems are known:
systems for generating analog signals using basic functions of ramp or logarithm type. These basic functions are obtained, by way of example, by using the knee voltage of one or more diodes,
systems having surface acoustic wave filters (SAW), these filters being followed by frequency transposition,
systems made up of a set of switched oscillators.
However, these solutions are not flexible and do not allow modification of the parameters of the signal that is generated. Moreover, when switched oscillators are used, the interferences between the oscillators damage the spectral purity of the generated signal.
In order to generate signals that have a predetermined form but whose parameters can evolve, it is known practice to use the following systems:
a system having a voltage controlled oscillator (VCO). This oscillator can be followed by a phase locked loop (PLL),
a system having a counter, a memory and a digital/analog converter, all of which is followed by a frequency transposition.
However, these systems are limited because the frequency spread band and the center frequency of the signal that is generated need to be in a limited range.
All of these devices can be integrated into a specialized programmable circuit, also known by the abbreviation “DDS” for direct digital synthesizer.
For signals that are not known in advance and need to be generated on request and in a very short time, the following systems are known:
digital radio frequency memory (DRFM) systems, which allow an analog signal to be stored in memory and this recorded signal to be reconstructed and retransmitted. These systems are particularly used in radars for generating signals that decoy other radars. However, these systems can only generate signals with a small dynamic range, the number of bits of dynamics actually being of the order of a few bits. This order of magnitude is of the order of 1 to 6 bits according to the frequency of the signal to be generated,
a sigma/delta converter, this type of converter being based on the principle of oversampling the input signal. This system then has a comparator using one bit to convert the difference between the input signal and the result of the conversion. The result of the comparison is used by a decimator filter, which sums the samples of the input signal. The sigma/delta converter is therefore a servosystem that uses input/output looping. This looping limits the bandwidth of the signal that can be generated.
It is also known practice to use a diode shaper, as shown in FIG. 1. In this system, a voltage ramp is sent to the terminals Vi and will generate a sinusoidal wave that is reconstructed by summation of trapezoids. However, this solution is limited in terms of frequency because the voltage to be generated, which will depend on the derivative of the voltage Vi(dVi/dt), is limited by the production technology used.